Method of coating a substrate

ABSTRACT

A method of coating a metal substrate with a protective metal coating by melting the coating, heating the surface only of the substrate with a high frequency induction heater to the melting temperature of the coating or greater and substantially immediately applying the molten metal coating to the heated substrate prior to penetrate of the heat through the substrate.

PRIORITY CLAIMS

This application is a continuation of, claims priority to and thebenefit of U.S. patent application Ser. No. 12/758,098, filed on Apr.12, 2010, which is a non-provisional of, claims priority to and thebenefit of U.S. Provisional Patent Application No. 61/169,330 filed Apr.15, 2009, the entire contents of each are incorporated by referenceherein.

FIELD OF THE INVENTION

This invention relates to a method of coating a metal substrate with aprotective metal coating having a melting temperature which issubstantially less than the melting temperature of the substrate. Forexample only, the substrate may be a steel or a ferrous metal tube,pipe, solid material, such as bar, beam, channel, plate or strip of anywidth or length and the protective metal coating may be zinc, aluminumor various alloys of zinc and aluminum or other protective coatingmaterials that adhere to an inductive substrate material, such as tin,silver, or copper. The method of this invention may be a continuousin-line process or the substrates may be individually coated in a batchprocess.

BACKGROUND AND SUMMARY OF THE INVENTION

In strip, bar and tube, in-line continuous coating systems, materialspeeds as high as 1000 feet per minute have been achieved successfully.In real time, the material will have traveled almost 17 feet in 1second. In the case of the tube coating, the length of time the steelsubstrate is in contact with the zinc after full material heating isabout 0.1 second, and into a water quench in less than 2 seconds.

The method of coating of this invention uses an electrical inductionmethod of heating the substrate, but rather than use 3000 hertz orless—as is common in the industry, the method of this invention uses ahigh frequency induction heater at frequencies such as 50,000 hertz orhigher (or lower where applicable, provided only the surface of thesubstrate is induction heated). The ultimate goal is to “surface heat”only the metal substrate such as steel, allowing the surface heat to beused in the molten coating metal, and then allowing for the conductivityof the remaining substrate to sync or soak the heat away from the moltencoating on the surface of the substrate. With the heat syncing of thesurface of the substrate, the protective metal coating cools quicklyallowing for the potential for thicker coatings than is currentlyavailable with the present coating methods.

The “surface heat” temperature required is a relative value dependentupon the molten coating material to be used. For example, the meltingtemperature of zinc is 787° F., usually applied at 850° F., but in themethod of this invention, the surface temperature may need to be as lowas the melting temperature of the protective metal coating or as high asup to 1000° F. or more. In the case of Aluminum as the protective metalcoating, the melting temperature is 1220° F., usually applied at 1350°F., but in the method of this invention, the aluminum coating may needto be applied at higher temperatures—up to 1500° F. The type of coatingmaterial can have a melting temperature as low as 449° F. for Tin, or ashigh as 1983° F. for Copper. Alloy versions of combination metals willhave various melting temperatures as well. In short, it is believed thatthe surface temperature needed should fall within 300° F. of the meltingtemperature of the molten protective metal coating material to beapplied. As known to those skilled in the art, many alloys of coatingmaterials, such as alloys of zinc and aluminum, have lower meltingtemperatures than either of the metals alone. For example, certainalloys of zinc, aluminum and magnesium have melting temperatures of 640°F. or less.

In an induction heating process, the frequency of the inductiondetermines the “depth of penetration” of the induction current throughthe substrate. The lower the frequency, the deeper the inductive heatpenetration—resulting in a more complete heating of the substrate. Asthe frequency increases, the heating depth of penetration decreases. Forexample, a 3000 Hertz frequency in mild steel has a depth of penetrationof roughly 0.042″. A 50,000 hertz frequency in the same steel wouldresult in a depth of penetration of roughly 0.007″. Higher frequenciessuch as 200,000 hertz would have a depth of penetration of 0.005 inches.Although the method of coating a metallic substrate of this inventionmay be used for coating various substrates, the most common substratefor this application is a ferrous metal substrate, particularly steeland the method of this invention will therefore be described as a methodof coating a steel substrate, but the method of this invention is notlimited to coating a steel substrate.

If the goal of the conventional induction heating is to completely heatthe steel substrate to a set temperature thru-out, as in the currentcoating methods, in most cases the lower frequency performs heating ofthe substrate much more efficiently. As the thickness of the steelsubstrate increases, the lower the frequency, the better the completeheating. Even then, the allowance of time for the heat to penetrate thesteel via conductive heat is required. From a costing standpoint, fullyheating the steel substrate with the low (3000 hertz or less)frequencies would be recommended.

The second part of the goal of this invention is twofold; first, toincrease the potential thickness of the protective metal coating as byheating and cooling the opposed sides of the substrate (e.g. theinterior and exterior of a tube), thus freezing the protective mealcoating faster, permitting a thicker coating. The second goal is thereare several alloys where the quick freezing of protective metal alloycreates a eutectic version of the alloy which has superior properties.

In the method of this invention, utilizing the minimum depth ofpenetration caused by the higher induction frequencies, allows for thesteel substrate surface, for example, to be heated, and thensubstantially immediately (preferably within about 0.1 or 0.2 seconds),the steel substrate would be either immersed into the molten protectivemetal coating or otherwise applied (sprayed, “rolled on”, etc.). Thedistance traveled in the fractional seconds of time will be dependent onthe mill speed, so distances and time frames will be relative. As soonas the steel leaves the molten coating metal, the coating thickness isadjusted by one of several available means, such as, but not limited to,“gas knives” or electrical controls if necessary.

As soon as the material leaves the molten metal, the “unheated” interiorportion of the substrate will begin heat syncing the residual heat fromthe heated surface, providing a relatively rapid heat loss at the coatedsurface area until the steel substrate is “equal in temperature”thru-out. At this point of the process, standard means of cooling can beused such as water, steam, air, most any inert or de-oxidizing gas, ortime to bring the temperature of the coated substrate down to allowabletemperatures.

In terms of “applicable frequency”, it is believed that the value willvary dependant on the wall thickness of the material to be coated andthe speed of the material being coated. In example, product “A” with awall thickness of 0.060 inches traveling at 300 feet per minute, a50,000 hertz value or higher would be recommended. Product “B” with awall thickness of 0.250″ traveling at 150 feet per minute, a 3000 hertzfrequency may be required. Product “C” with a thickness of 0.030″traveling at 800 feet per minute may need a frequency of 200,000 hertz.In one preferred embodiment, the metal substrate is heated with aninduction heater at a frequency of 10,000 hertz or greater.

Included in this concept, is the ability to control the liquid coatingmaterial temperature just before the steel substrate contacts theliquid. It is important to any plating or coating process to completeclean the surface to be coated and remove oxides, debris or soil andwater. In the process of this invention, the surface to be coated may becleaned by an acid bath or wheel abrading to remove oxides, then washedwith an alkali and dried or any conventional cleaning process.

The primary advantages of the method of this invention are as follows:

-   -   The energy requirements to heat just the surface of the steel        substrate are a relative constant regardless of wall thickness        of the substrate. As the wall thickness increases, the heating        costs do not. With the existing full heating process, the cost        of heating goes up linearly as the wall thickness increases        assuming speed stays the same. Thus, the method of this        invention is particularly advantageous with thicker-walled pipe        and plates. This will discussed further below.    -   The rapid cooling of the substrate after leaving the molten        metal bath will eliminate the need for the additional alloys of        the newer coating alloys, the time needed to cool—thus either        allowing existing coating systems to either speed up the        operation—increasing productivity, decrease the tower height or        for newer installations to have smaller cooling towers.    -   The rapid cooling of the coated part resulting from the fact        that only the surface is heated will also decrease operational        costs because of the reduced energy needed to cool the material        down.    -   Due to the quick freeze of the coating, improved surface        thickness variation may be achieved.    -   Better overall heat control of both the substrate and the        coating metal bath can be achieved versus standard gas fired        type of heating systems.    -   The higher the frequency, the quieter the induction unit. At        50,000 hertz for example is almost impossible to hear.    -   Wall thickness in standard coating systems can have significant        variation that would normally affect the coating process. Wall        thickness variation will not be affected with our process.    -   The method of this invention can vary the coating process to        include coating on one side, both sides, or differential        coatings on opposed sides.    -   With tubular coating systems, the elevated temperatures of the        steel substrate requires specialized paints that can handle the        extreme temperatures. With the process of this invention, the ID        temperature will be reduced opening the opportunity for        alternative—better and less costly paints.    -   Removal of the excess coating in atmosphere will reduce dross        and oxides significantly.

Process Example: Theoretical calculations were made for a steel tuberunning on a continuous galvanizing line (A “Krengel Flo-Coat” line).Using the real time induction heating software, it was determined howfast a steel substrate would heat, and how fast it would begin to “heatsync” or “soak” internally after starting the heating process.

-   -   The method of this invention also significantly reduces the        energy cost; not only because the energy required to heat only        the surface of the substrate is reduced, but also because the        molten protective metal coating with the method of this        invention is applied at near the freezing temperature of the        coating.

Using 3 test products and changing only the wall thickness to determinepower requirements and temperature changes the following was determined.

Running a 3.00 inches diameter tube at 300 feet per minute, the amountof travel in 1 second is 60″. 1/10^(th) of a second would therefore be 6inches.

With a 0.063 inches wall thickness, the surface would be heat to 850° F.in less than 0.1 seconds with an ID temperature of 351° F. 0.1 secondslater, the surface temperature would drop about 50° F. In another 0.1seconds, the temperature will have dropped an additional 250° F. (NOTE:This computer model does not take into account the ramifications of themolten metal adding to the total heating (or cooling) process. This willbe determined through actual testing. In roughly 0.25 seconds from thestart of the heating process, the steel substrate will have “synced” atroughly 5300 F, at which point conventional cooling methods will need tobe used. At 50,000 hertz, the power requirement would be 650 Kw. Usingstandard 3000 hertz induction heating, the power requirement would be1000 Kw. Economically, the electrical use requirements would be 65% forthe higher frequency versus the lower frequency.

With a 0.100 inch wall thickness, the surface would again heat to 8500 Fin less than 0.1 seconds with an ID temperature of 142° F.; 0.1 secondslater, the surface temperature would still only drop about 50° F. Inanother 0.1 seconds, the temperature will have dropped an additional3800 F. (NOTE: This computer model does not take into account theramifications of the molten metal adding to the total heating or coolingprocess. This will be determined through actual testing). In roughly 0.3seconds from the start of the heating process, the steel substrate willhave “synced” at roughly 370° F., at which point conventional coolingmethods will need to be used. At 50,000 hertz, the power requirementwould be 714 Kw. Using standard 3000 hertz induction heating, the powerrequirement would be 1824 Kw. Economically, the electrical userequirements would be only 39% for the higher frequency versus the lowerfrequency.

With a 0.120 inch wall thickness, the power increase using 50,000 hertzwould be only 6 kw higher at 720 Kw, while the conventional 3000 hertzprocess would need 2244 Kw. Economically, the electrical userequirements would be 32% for the higher frequency versus the lowerfrequency.

Briefly, the method of this invention includes melting the protectivemetal coating and heating the surface only of the substrate with a highfrequency induction heater to a temperature equal to or preferablygreater than the melting temperature of the protective metal coating.The method then includes substantially immediately applying the moltenprotective metal coating to the heated surface of the substrate prior topenetration of the heat through the substrate and then freezing theprotective metal coating on the substrate. The method of this inventionmay be used to coat both sides of a metal plate, for example, bysimultaneously heating the surface of both sides of the plate andapplying molten protective metal coating to both sides prior topenetration of the heat through the plate by immersing the plate inmolten protective metal coating, for example. In one preferredembodiment, the molten protective metal coating is applied within 0.3seconds following heating or 0.1 seconds depending upon the factorstated above.

In the disclosed embodiment, the surface of the metal substrate isheated with an induction heater at a frequency of 10,000 hertz orgreater. The protective metal coating may be applied to the heatedsurface of the metal substrate prior to fifty percent penetration of theheat through the substrate or prior to thirty percent penetration. Asstated above, the metal substrate may be a ferrous metal substrate, suchas steel and the protective metal coating may include zinc, aluminum oralloys thereof. As will be understood by those skilled in this art,various modifications may be made to the following disclosed embodimentof this invention within the purview of the appended claims. By way ofexample only, the method of this invention is equally application tocoating solid bars and sheet metals.

BRIEF DESCRIPTION OF THE DRAWING

The drawing and following description of a preferred embodimentillustrates the method of this invention coating a tube, piper or strip,such as a horizontal tube, wire, narrow strip, or similar productapplications. The end product can be any size or shape as long as it canbe run in a continuous line. The concept will easily work for verticalor any angle situations just as well, and in fact, for products such aswide steel strip would be preferred.

FIG. 1 is a side view, partially cross-sectioned of a pipe or tube beingcoated by the method of this invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 represents the “tube or pipe” version of the method of thisinvention, but the disclosed method can be used for wire, rod, narrowstrip, or other materials or substrate 21 to be coated in line or batch,referred to herein as the material or substrate. The method illustratedis only one of many potential ways the process can be performed. Again,the substrate to be coated may be vertical, horizontal or any otherangle. In the disclosed embodiment of the method of this invention, thesubstrate is substrate 21 is moving. The material or substrate 21 to becoated is moving in FIG. 1 from right to left. The protective metalcoating material to be applied, e.g.: zinc, aluminum, their alloys usedin coating applications, including other metals as described above is tobe referred to as “protective coating material” 25.

For example, a 3.00 inch Diameter Tube×0.0625″ Wall Thickness @ 300Feet/Minute. Temperatures and time values are computer-generatedestimates. Estimates do not consider any additional heat the coatingmaterial 25 generates at the surface when in contact with the substrate21.

The cleaned, prepared material 21 to be coated travels into an enclosure27. The substrate material 21 immediately enters into the coatingchamber 24. It is also important in this process to completely fill thecoating chamber 24 with molten protective metal coating material andmaintain the coating chamber 24 completely full of molten metal coating.The purpose of the enclosure 27 is to enclose the substrate duringheating and coating to avoid oxidation. The enclosure preferably has anon-oxidizing atmosphere, such as nitrogen or a reducing agent, such ashydrogen. It is believed, however, that a non-oxidizing atmosphere maynot be required for all applications. The collars 23 at the entry andexit of the enclosure may be required to contain the non-oxidizing gasin the enclosure 27.

The coating chamber 24 may be made of a non-electric/high temperaturematerial capable of coming into contact with the coating material 25without problems. There are many types of refractoriness that arecapable of being used. In the disclosed embodiment, the High Frequency(HF) work coil 22 surrounds the coating chamber at the entry end to heatthe substrate material 23 to the necessary alloy temperature needed. Aswill be understood by those skilled in this art, where the method ofthis invention is used to surface heat a plate or strip, the inductioncoil may not be able to surround the plate or strip. Instead, a coilwhich only heats one side of the plate may be used. For example, agenerally sinusoidal-shaped HF coil may be used adjacent one side of theplate may be used. A roller opposite the induction coil may also beuseful to oppose the force of the flux of the induction heater. It isalso possible to cool the opposite side of the plate to substantiallyimmediately freeze the coating. The necessary alloy temperature can varysignificantly from metal to metal alloy. Higher or lower substratesurface temperature may vary as well. For the purpose of thisdisclosure, the term alloy temperature needed or required temperaturemeans the temperature required for heating the surface of the substrateto at or above the melting temperature of the protective metal coatingas disclosed herein. The coupling gap is critical to the efficiency ofthe HF induction power, so the coating chamber 27 area between the tube21 and the coil 22 will be relatively close. The HF work coil 22integral with the coating chamber 24 mayor may not be required for allapplications, especially where the enclosure chamber 27 is filled with anonoxidizing atmosphere. Further, the enclosure 27 may not be requiredif a non-oxidizing gas is pumped into the entry collar 23. There may, ormay not, be a substrate material 21 stabilizing roll or fixture toassure the coupling of the HF work coil 22 to the material 21 staysconstant.

As the substrate material 21 leaves the high frequency (HF) work coil 22area, the coating chamber 24 is sized to allow free flowing moltencoating metal 25 to surround the substrate material 21 being coated. Thesurface temperature of the material 21 as it leaves the HF work coil isof sufficient temperature as to allow for alloying to the coatingmaterial 25, but because the surface temperature will decrease extremelyquickly, the distance from the HF coil to the coating material 25 isimportant. The length of the coating chamber 24 will be whatever lengthis necessary for the alloying to take place, but a longer length willnot hurt the process. The application of the coating material 25 is notlimited to a system as shown above. it is also important with the methodof coating of this invention to control the temperature of the moltenprotective metal coating. The disclosed apparatus should include atemperature control which maintains the temperature of the molten metalcoating preferably within plus or minus one percent. The protectivemetal coating can be also be sprayed, rolled, or applied in one ofvarious ways.

One important element for the method of this invention is the issue oftime and temperature, getting the material 21 to a necessary surfacetemperature with a “high enough” frequency, and having the material 21come into contact with the molten coating material 25 as quickly aspossible before losing the surface heat through conductivity into thebody of the substrate material 21.

As the now coated substrate material leaves the coating chamber 24, anexcess coating “wiping system” such as an air ring, magnetic lines offlux, or any other type of system 26 removes excess coating material 25leaving a controlled coating thickness on the material 21. It ispreferable that whatever wiping system is used, it is done within thecontrolled atmosphere, but this is not necessary because once thecoating material 25 is alloyed to the steel substrate, the need for acontrolled atmosphere is gone. That said, as long as the wiping systemis in atmosphere, the amount of metal oxide generated is minimized.

As stated above, one advantage of the method of this invention is thatthe surface of the substrate opposite the induction heated surface maybe painted with a coating, such as a water borne paint, Teflon or thelike, which would burn or char in a conventional induction heatingprocess where the surface is heated with a conventional low frequencyinduction heater. For example only, the inner surface of a steel pipemay first be coated or painted with a Teflon coating having a relativelylow char temperature and the outer surface of the pipe may then beheated with a high frequency induction heater and coated with a moltenzinc, aluminum or their alloys, as described above, without adverselyaffecting the Teflon coating on the inner surface of the pipe. In aconventional low frequency induction heating process, the low frequencyinduction heat would soak through the pipe and destroy the Tefloncoating on the inner surface of the pipe.

Included in this concept, is the ability to control the liquid coatingmaterial 25 temperatures just before the steel substrate contacts theliquid—as it is pumped into the chamber 24.

As will be understood from the above description of the method ofcoating a metal substrate with an adherent protective metal coating ofthis invention, an important element of the process is the issue of timeand temperature. Heating the surface of the metal substrate to anecessary surface temperature with a high frequency induction heater andsubstantially immediately applying the molten protective metal beforethe heat “soaks” through the substrate is very important to achieve theadvantage of this invention. It is also important to control thetemperature of the molten protective coating just before applying thecoating to the heated substrate.

The invention is claimed as follows:
 1. A method of coating a metalsubstrate having a first outer surface and a second inner surface, saidmethod comprising the following steps performed in a continuous inlineprocess: melting an adherent metal coating in a coating chamber andmaintaining said melted metal coating in a melted form such that themelted metal coating is maintained directly adjacent to an inductionheating coil exit end of an induction coil, said metal coating having amelting temperature substantially less than a melting temperature of themetal substrate; applying a paint to the second inner surface of themetal substrate, said paint having a burning temperature, said burningtemperature of the paint being less than said melting temperature of themetal coating; thereafter, heating the first outer surface of the metalsubstrate with the induction heating coil at a frequency of at least10,000 Hertz to a temperature equal to or greater than the meltingtemperature of the metal coating, and without actively heating thesecond inner surface of the metal substrate and without adverselyaffecting the paint on the second inner surface of the metal substrate;immediately after heating the first outer surface of the metal substratewith the induction heating coil, moving the heated first outer surfaceof the metal substrate out of the induction coil exit end and directlyinto the directly adjacent melted metal coating in the coating chamberand applying the molten metal coating on the heated first outer surfaceof the metal substrate prior to penetration of the heat through themetal substrate to the second inner surface of the metal substrate bymoving the metal substrate through the molten metal coating; andimmediately after applying the molten metal coating to the heated firstouter surface of the metal substrate, quenching the molten metal coatingon the first outer surface of the metal substrate with a fluid coolanthaving a temperature substantially below the melting temperature of themetal coating, thereby freezing the molten metal coating on the firstouter surface of the metal substrate to have a predetermined thickness.2. The method of claim 1, which includes using the induction coil toheat the first outer surface of the metal substrate with the inductionheating coil at a frequency of at least 50,000 Hertz.
 3. The method ofclaim 1, which includes applying the molten metal coating on the heatedfirst outer surface of the metal substrate prior to fifty percentpenetration of the heat through the metal substrate toward the secondinner surface of the metal substrate.
 4. The method of claim 1, whichincludes applying the molten metal coating on the heated first outersurface of the metal substrate prior to thirty percent penetration ofthe heat through the metal substrate toward the second inner surface ofthe metal substrate.
 5. The method of claim 1, wherein the metalsubstrate is a ferrous metal substrate.
 6. The method of claim 1,wherein the metal coating includes zinc.
 7. The method of claim 1, whichincludes applying molten metal coating on the heated first outer surfaceof the metal substrate within 0.2 seconds following heating of the firstouter surface of the metal substrate.
 8. The method of claim 1, whichincludes applying molten metal coating on the heated first outer surfaceof the metal substrate within 0.1 second following heating of the firstouter surface of the metal substrate.
 9. The method of claim 1, whereinthe metal substrate is tubular, the first outer surface is an outersurface of the tubular metal substrate, and the second inner surface isan inner surface of the tubular metal substrate, and which includesheating the first outer surface of the metal substrate with theinduction heating coil at a frequency of at least 50,000 Hertz, applyingthe molten metal coating on the first outer surface of the metalsubstrate prior to fifty percent penetration of the heat through themetal substrate toward the second inner surface of the metal substrate,and applying molten metal coating on the heated first outer surface ofthe metal substrate within 0.1 seconds following heating of the firstouter surface of the metal substrate.
 10. A method of coating a metalsubstrate having a first outer surface and a second inner surface, saidmethod comprising the following steps performed in a continuous inlineprocess: melting an adherent metal coating in a coating chamber andmaintaining said melted metal coating in a melted form such that themelted metal coating is maintained directly adjacent to an inductionheating coil exit end of an induction coil, said metal coating having amelting temperature substantially less than a melting temperature of themetal substrate; applying a polytetrafluoroethylene to the second innersurface of the metal substrate, said polytetrafluoroethylene having aburning temperature, said burning temperature of thepolytetrafluoroethylene being less than said melting temperature of themetal coating; thereafter, heating the first outer surface of the metalsubstrate with the induction heating coil at a frequency of at least10,000 Hertz to a temperature equal to or greater than the meltingtemperature of the metal coating, and without actively heating thesecond inner surface of the metal substrate and without adverselyaffecting the polytetrafluoroethylene on the second inner surface of themetal substrate; immediately after heating the first outer surface ofthe metal substrate with the induction heating coil, moving the heatedfirst outer surface of the metal substrate out of the induction coilexit end and directly into the directly adjacent melted metal coating inthe coating chamber and applying the molten metal coating on the heatedfirst outer surface of the metal substrate prior to penetration of theheat through the metal substrate to the second inner surface of themetal substrate by moving the metal substrate through the molten metalcoating; and immediately after applying the molten metal coating to theheated first outer surface of the metal substrate, quenching the moltenmetal coating on the first outer surface of the metal substrate with afluid coolant having a temperature substantially below the meltingtemperature of the metal coating, thereby freezing the molten metalcoating on the first outer surface of the metal substrate to have apredetermined thickness.
 11. The method of claim 10, which includesusing the induction coil to heat the first outer surface of the metalsubstrate with the induction heating coil at a frequency of at least50,000 Hertz.
 12. The method of claim 10, which includes applying themolten metal coating on the heated first outer surface of the metalsubstrate prior to fifty percent penetration of the heat through themetal substrate toward the second inner surface of the metal substrate.13. The method of claim 10, which includes applying the molten metalcoating on the heated first outer surface of the metal substrate priorto thirty percent penetration of the heat through the metal substratetoward the second inner surface of the metal substrate.
 14. The methodof claim 10, wherein the metal substrate is a ferrous metal substrate.15. The method of claim 10, wherein the metal coating includes zinc. 16.The method of claim 10, which includes applying molten metal coating onthe heated first outer surface of the metal substrate within 0.2 secondsfollowing heating of the first outer surface of the metal substrate. 17.The method of claim 10, which includes applying molten metal coating onthe heated first outer surface of the metal substrate within 0.1 secondsfollowing heating of the first outer surface of the metal substrate. 18.The method of claim 10, wherein the metal substrate is tubular, thefirst outer surface is an outer surface of the tubular metal substrate,and the second inner surface is an inner surface of the tubular metalsubstrate, and which includes heating the first outer surface of themetal substrate with the induction heating coil at a frequency of atleast 50,000 Hertz, applying the molten metal coating on the first outersurface of the metal substrate prior to fifty percent penetration of theheat through the metal substrate toward the second inner surface of themetal substrate, and applying molten metal coating on the heated firstouter surface of the metal substrate within 0.1 seconds followingheating of the first outer surface of the metal substrate.